Solution.java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public boolean checkSubTree(TreeNode t1, TreeNode t2) {
        if (t2 == null) {
            return true;
        }
        if (t1 == null) {
            return false;
        }
        if (dfs(t1, t2)) {
            return true;
        }
        return checkSubTree(t1.left, t2) || checkSubTree(t1.right, t2);
    }

    private boolean dfs(TreeNode t1, TreeNode t2) {
        if (t2 == null) {
            return t1 == null;
        }
        if (t1 == null || t1.val != t2.val) {
            return false;
        }
        return dfs(t1.left, t2.left) && dfs(t1.right, t2.right);
    }
}